It is well known to employ self-propelled percussion tools for making small diameter horizontal holes through soil. Typically, these pneumatic, impact or percussion boring tools are used to displace soil in order to create horizontal holes beneath roadways to lay pipes or cables without having to dig a trench across the roadway.
These pneumatic boring tools generally include a torpedo-shaped body or casing having a tapered front nose portion and a hammer piston or striker disposed in a cavity provided in the casing for reciprocal movement therein. The hammer impacts against a front wall (front anvil) forming a portion of the cavity of the tool casing, causing the tool to move incrementally forward into the soil. The friction between the outside of the tool body and the surrounding soil tends to hold the tool in place as the striker reciprocates in the cavity for another impact against the front wall.
Generally, these percussion burrowing tools include an air hose extending from the tail assembly to provide pressurized air for pneumatic operation of the striker. During each piston cycle, air is typically exhausted from a rear opening at the tail assembly into the hole created by the boring tool. The valve mechanism inside the tool permits the pressurized air in the hose to be directed to drive the striker in a forward direction (i.e., to strike the front anvil), and also for driving the striker in a reverse direction (i.e., to strike a rear wall or rear anvil of the cavity). During each complete stroke of the piston hammer, there is a force stroke wherein pressurized air moves the piston against the front or rear anvil, and a return stroke wherein pressurized air returns the piston to an initial position. Typical of these patented pneumatic ground piercing tools are disclosed in U.S. Pat. Nos. 3,410,354; 3,756,328; 4,078,619; 4,221,157; 4,609,052; 4,662,457; and 5,025,868.
Reversing the direction of the hammer piston or striker to reverse the direction of incremental movement of the boring tool for recovery can be accomplished employing a number of different mechanisms and methods. Generally, however, in each device, the compressed air is communicated between an internal control sleeve or valve and the reciprocating hammer at two different locations within the device to cause the striker to impact either the front anvil or the rear anvil. Typical of these patented structures are disclosed in U.S. Pat. Nos. 3,616,865; 3,651,874; 3,705,633; 3,727,701; 3,744,576; 3,756,328; 3,763,939; 3,995,702; 4,078,619; 4,121,672; 4,132,277; 4,171,727; 4,284,147 and 4,662,457.
While reversing the direction of the striker has been adequate to recover these boring tools out of generally horizontal holes, the application of this technique to recover the tool from near vertical holes is not nearly as effective.
Soil conditions sometimes arise where the soil begins to collapse and fall into the vertical shaft atop the tail assembly of the tool. As the soil collapses atop the tail assembly, the boring tool, in the reverse direction, begins to compact the soil from below and is unable to push its way upwardly. Hence, the recovery of these boring tools situated in vertically deeper holes, sometimes several times deeper than the length of the tools, is increasingly difficult. Further, the tools own weight in addition to the weight of the collapsed soil, and the generally blunt rear end of the casing, reduce the efficiency and effectiveness necessary to recover the tool.
Moreover, as the collapsed soil builds up atop the rear of the boring tool, the air exhausted from the rear of the tail assembly may be impeded. This causes blockage of the air flow which, as a result, reduces the power output of the tool. Ultimately, the reversing process may stop, leaving the boring tool wedged under the collapsed soil and unrecoverable under its own power. In this situation, it may be necessary to dig the boring tool out of the hole which is a time consuming and costly process or abandon the tool.
Typically, only the air hose extends rearwardly from the end of the boring tool. Pulling the boring tool by the air hose, in addition to overcoming the frictional forces of the wall and the weight of the collapsed soil, would most probably sever the hose.
One technique applicable towards the recovery of the vertical boring tool is that provided by the GRUNDOMAT.RTM. vertical bore machine which employs a percussion tool having a plurality of segmented steel pipes attached to the back of the tool. Each segment is added to the end of the last segment as the pipe descends below the ground into the hole. The steel pipe segments have a diameter substantially the same as the tool casing which further prevents caving of the vertical hole walls.
Problems occur, however, when attempting to recover the boring tool from deeper holes. The combined weight of the tool and segmented pipes, and more importantly, the frictional drag caused by the engagement of the perimeter walls of the pipe with the vertical hole walls substantially reduce the effectiveness of self-propelled recovery.
Accordingly, the GRUNDOMAT.RTM. vertical bore machine often requires the employment of a tall crane or hoist assembly vertically positioned over hole. This crane is formed to grip the end of the pipe to pull the boring tool apparatus out of the hole by mainly overcoming the frictional drag between the segmented pipe and the vertical hole walls. Not only is this procedure time consuming and costly to operate, however, some applications are limited or precluded where the vertical spacing above the hole is inadequate to install the crane or hoist assembly.
Another approach generally applied in softer soils has been to place a substantial portion of the boring tool inside a nose cone which is bored (pushed) into the ground as the boring tool pounds against the inner front walls of the nose cone. Subsequent sections of pipe are then attached to the rear of the front nose cone section. The nose cone and segmented pipe are then retained in the vertical hole, while the boring tool is pulled up and out of the nose cone and segmented pipe.
This technique is not capable of boring into harder or more compacted soil since the boring tool looses substantial efficiency inside the cone as some of the power is absorbed in the cone itself. Normally, the boring tool relies on the skin friction between the ground and the casing exterior surface to "push" off of the soil to further drive the boring tool into the ground. Since the boring tool is only loosely disposed in the nose cone, the boring tool has nothing to frictionally abut or push-off against.